Water Ponding & Drainage
First, let’s take up the definition of the term ponding water:
“It’s standing water that takes more than 48 hours to drain or evaporate off the roofing system.”
If it rains for a week straight you will have water on the roof while it is raining but that is different than a pool of water sitting in one area and taking days or weeks to dry out. Some roofs have standing water at all times with the source of the water usually from A/C units that drain onto the roof instead of being piped off. Regardless of the water source, ponding conditions affect the roofing system and are quite often the source of roof failure.
One of the solutions I see on roofs is the addition of an extra layer of roofing material on top of the ponding location but the ponding is still there. Another is just to leave the ponding condition alone. The best solution is to build-up the depression which enables the water to drain off the roof but this is not often done by most roofing companies.
When a new roof is installed correctly it will hold up to ponding conditions but only for the short term of 1-5 years. The ponding gradually starts to deteriorate the roofing material.
Ponding areas will have a lot of sediment collect within it. This sediment is, in part, pollutants, dust particles and debris. We often find plants growing in these areas as well, an obvious sign that the roof has had little or no maintenance. The roofing material in these ponding areas will rot away.
Your roof lasts much longer when you have good drainage. A repair over ponding areas which doesn’t correct the depression is an invitation for future leaks.
Common reason for roof leaks are ponding conditions.
Some causes of ponding:
- Buildings settle over time, older buildings tend to have more ponding. This is the most common reason on older buildings
- Rotted wood beneath the roofing that has caved in but was repaired or roofed over (rotted wood is usually from a past leak)
- Rafters and beams that have been widely spaced allowing the sheathing to sag-too little support; this is the second most common reason
- Earthquake damage where a support has given way allowing it to sag
- Poor roofing practices where drains have been installed higher than the roofing material
- Thick tar repairs which then holds back water; small dams are created
- Construction where drainage was not well designed into the building-less than ¼
- Heavy objects on the roof such as A/C equipment, etc., loaded onto a lightly framed roof and which cannot support such a weight creates sagging in the wood substrates
- Pipes or conduits lying on the roof's surface-won't allow water or any debris to easily flow under
- Buildup of trash, leaves, construction materials and roof granules
- Too many layers of roofing - this will distort drainage and hold back water
- Thick repair patches in drainage areas
Commercial Cool Roof Systems has a proprietary light weight leveling compound specially made for roofing which alleviates ponding conditions.